A nuclear reactor comprises a core of fissionable fuel which generates heat during fission. The heat is removed from the fuel core by the reactor coolant, i.e. water, which is contained in a reactor pressure vessel. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions. As used herein, the term "high-temperature water" means water having a temperature of about 150.degree. C. or greater, steam, or the condensate thereof.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC. The present invention is concerned with mitigating welding-induced residual stresses and thermal sensitization, which can lead to SCC in susceptible metals.
A conventional V-groove weld 6 for joining two pipes 2 and 4 is illustrated in FIG. 1A. The weld 6 is formed by filling the V-groove with beads of fused material from a filler wire placed at the tip of a circular cylindrical welding electrode (not shown). This welding process produces a very wide heat affected zone (HAZ) in the vicinity of the welded joint. The occurrence of SCC in the vicinity of such welded joints has led to the need for repair or replacement of much of the piping in light water reactor power plants throughout the world. Numerous methods have been utilized for over a decade to improve the tensile residual stress state in the vicinity of welded joints, including magnetic induction, electrical resistance and electric arc heating methods. All of these methods are based on generating a substantial temperature difference through the welded material thickness by applying the heat source on one side of the material and maintaining water cooling on the other side of the material. This temperature difference produces thermal strains and subsequent material plasticity, and a corresponding stress reversal across the thickness of the material. The net result makes the residual stress on the side of the joint exposed to the potentially aggressive reactor water environment significantly less tensile or, more preferably, compressive. These previous methods, including "heat sink welding" and "last pass heat sink welding" have all relied on continuous water convective cooling of the environmentally exposed side of the weld joint in order to effect the required temperature difference and stress reversal. This water cooling requirement is a severe penalty to the fabricator whether the piping is being newly installed or replaced, since the complete piping system must be intact in order to contain the water. The typically used arc welding process which requires water cooling to effect the temperature gradient through the material thickness and a corresponding residual stress reversal has relatively low thermal and time efficiencies and utilizes a wide weld joint design with a low aspect ratio of joint depth to thickness. The reduction of tensile forces residing in the metal lattice structure by internal water cooling during welding serves to mitigate the occurrence of irradiation-assisted SCC, wherein impurities in the stainless steel alloy diffuse to the grain boundaries in response to the impingement of neutrons.
A second major contributor to SCC in stainless steels alloyed with chromium for corrosion resistance is the size and degree of thermal sensitization of the heat affected zone adjacent to the weld. Thermal sensitization refers to the process whereby chromium carbides precipitate in the grain boundaries of the material. The precipitation of chromium carbides ties up the chromium which would otherwise be in solution. Thus, a thin layer along the grain boundary is denuded of chromium, creating a zone which is no longer corrosion resistant and therefore is susceptible to SCC. Such stainless steels corrode at the grain boundaries preferentially.
One consideration in the design of welds for SCC resistance is the minimization of the heat input by the process to the component being joined. This heat input is typically maintained at a level sufficient to provide reliable fusion by the weld filler metal to the side walls of the joint, which have in other welding processes been separated by an amount necessary to move a circular cylindrical electrode in the joint.
One type of reduced-groove-width welding process used commercially in power plant piping welds is so-called "narrow groove" welding, an illustration of which is given in FIG. 1B. This technique produces a weld 6' between pipes 2' and 4' which has a heat affected zone which is narrower than and a groove angle which is less than the HAZ and groove angle of the V-groove welding process. The "narrow groove" welding process uses a standard circular cylindrical electrode geometry. These standard electrodes come in various lengths and diameters, typically with a relatively pointed or conical end. However, in "narrow groove" welding, the reduction of the groove width has been limited by the minimum diameter of the electrode required to reliably carry the needed welding current. All previous welds, including "narrow groove" welds, have been made with the circular cylindrical electrode shape, which has become the industry standard. The minimum diameter of a circular cylindrical electrode is in turn limited by the electrical current-carrying and heat-dissipating capability of a given size. No provision has ever been made for the manufacture or installation of a noncylindrical electrode in either a V-groove or "narrow groove" weld application.